This invention relates to an optical-fibre T-coupler comprising solid-glass cores and reflective systems. Fibre-optics T-couplers are already known in embodiments including guide members and also provided with reflective arrangements. For example, in U.S. Pat. No. 3,883,217 there are described T-couplers in which two solid-glass cores arranged in series are interposed between the terminal surfaces of an interrupted primary fibre bundle and have the light guides of the coupled branches (receiver/transmitter) and also a direct connection applied to their inner surfaces. The incoming light signal is thus divided into a component led to the receiver and a component passing directly through the coupler. The light passing directly through the coupler and the transmitter signal injected in the same direction combine into a new transmitted signal at the coupler output. The two series-arranged solid-glass cores serve the function of a diffusor (randomiser, scrambler). Beyond a certain length of core, light entering at any point is distributed over the entire cross-sectional area of the core and thus passes uniformly into all the fibers of the outgoing fibre bundle. The relatively complex construction is a disadvantage. Six individual components are present in the device, of which the end faces must be exactly aligned and cemented. Even the construction of the fibre bundle providing the direct connection is not without problems.
In accordance with U.S. Pat. No. 3,870,396 right-angled isosceles glass prisms are interposed in the primary fibre bundle and the hypotenuse surfaces, positioned so as to be inclined relative to the direction of the transmitted light, are bridged by a reflector. This reflector is only partly reflective at the position at which the receiver signal is coupled out and is fully reflective at the position for the introduction of the transmitter signal. The arrangement is directional, so that two such couplers are necessary for duplex operation. The large coupling cross-sections which must subsequently be reduced, by way of guide transition sections (naturally accompanied by radiation losses or reflections) to the cross-section of the outgoing fibre bundle are a disadvantage. Exact adjustment of the individual coupling conditions at the partly transmissive reflector can also be far from simple.
A principal disadvantate of the T-coupler is known to be the relatively high transmission attenuation of the order of 3-4 dB resulting from the losses through the spacer and sheathing cross-section of the outgoing fibre bundle. Applications may be envisaged, however, in which these characteristics are of less importance.